1. Field of the Invention
The present invention relates to a process and apparatus for controlling harmful organic emissions from waste gas vent streams. More particularly, this invention relates to a method and system for reducing emissions of volatile organic compounds and hazardous air pollutants from natural gas dehydrators.
2. Description of the Prior Art
Processing of natural gas includes the removal of water and other contaminants to provide a uniformly dry gas for industrial and consumer use. It is essential that water from produced natural gas streams be removed to prevent ice formation and corrosion in gas pipelines, valves, and fittings. The most widely used water removal or dehydration units in the natural gas industry are those based on water absorption by ethylene glycol, diethylene glycol or triethylene glycol.
Glycol dehydration systems have been the subject of prior patents such as U.S. Pat. Nos. 4,182,659 and 4,314,891. As described therein, these dehydrators basically include an absorption column, a pump to move the glycol through the system, and a glycol reboiler to regenerate the glycol. Most such systems also employ a three-phase condensate separator to recover some of the immiscible hydrocarbons from the glycol prior to drying the glycol in the glycol reboiler. In the absorption column, the wet gas stream contacts glycol in a countercurrent manner, wherein water and various hydrocarbon impurities are absorbed by the glycol. The water-containing glycol solutions then pass to the glycol reboiler or regenerator, where the greater part of the water is vaporized in the form of steam and separated. A still column is usually located on top of the reboiler to separate the water and glycol. A reflux coil or other reflux mechanisms may be utilized to condense vapor in the still column. Concentrated glycol having a low water content is then recovered and recycled for further use in drying the gas.
Emissions consisting largely of water vapor are vented to the atmosphere from the still column of the glycol reboiler. These emissions also contain toxic compounds such as benzene, toluene, ethylbenzene, and xylenes (hereinafter referred to as "BTEX") and other volatile organic compounds ("VOC's"), all of which serve as a source of air pollution in the absence of any control system to recover or destroy these organic species. In fact, the 1990 Clean Act Amendments enumerated a number of these species as being "Hazardous Air Pollutants" and prescribed certain levels for their control. This recent legislation alerted the oil and gas industry to a previously unrecognized problem regarding emissions to the air from systems designed to remove water from natural gas streams.
Various control systems have been tried by the natural gas industry for the purpose of minimizing organic emissions from glycol dehydrators in compliance with air pollution laws. The most common systems utilize heat exchangers to condense and recover some of the VOC's. These condensers often use air as the cooling medium, since glycol dehydrators are frequently located in remote areas having no available cooling water. Condensers installed in the past were usually not designed such that they can comply with the requirements of the 1990 Clean Air Act Amendments. Additionally, thermal and catalytic incineration techniques have been considered by the industry as solutions to the pollution problem associated with organic emissions, but were found to be too costly to implement and fraught with technical and safety problems. Moreover, incineration wastes a valuable hydrocarbon resource and adds undesirable carbon dioxide to the atmosphere without producing any commensurate benefits.
The primary systems presently used in the industry to control emissions of the forgoing type are tubular air and/or water-cooled condensers. These condensers produce a vent gas, an organic liquid phase, and a water phase. While these systems substantially reduce BTEX emissions, a significant amount of valuable hydrocarbons continue to be annually emitted to the atmosphere. Moreover, a highly contaminated water stream is produced by such control systems, which may require treatment prior to use or disposal.
U.S. Pat. Nos. 5,209,762 and 5,346,537 to Lowell disclose improvements in conventional glycol dehydration systems which enables control of emissions of volatile organic compounds, while minimizing the contaminants in the water streams produced. In U.S. Pat. No. 5,209,762, vented steam and gaseous VOC's from the glycol regenerator are flowed as input to the bottom of a BTEX steam stripper column, and vapors from the top of the steam stripper column are flowed to a cooling condenser to produce a water stream having a relatively high content of organic compounds. The water stream is passed to the top of the steam stripper and then through the stripper in counter-current relation to the vented gases and steam, whereby a stripped water stream having a low content of organics emerges from the bottom of the stripper column. While this system is effective for its intended purposes, it is technologically complex and relatively costly to implement.
In U.S. Pat. No. 5,346,537, a simplified modification of the aforementioned system is described in which the BTEX steam stripper is eliminated. This simplified system relies on a cooling tower to air strip organics from the condenser water stream. Vented steam and volatile organics from the glycol reboiler are input to an air-cooled condenser and a downstream water-cooled condenser connected in series to produce a condensed water stream having dissolved organic compounds, an organic vent gas, and an organic liquid. These phases are separated in an accumulator. The condensed water stream is passed through a cooling tower in counter-current relation to stripping air, whereby a stripped water stream having a low content of organics emerges from the tower. Like the system described in U.S. Pat. No. 5,209,762, this modified system is complex, costly, and emits to the air the VOC's stripped from the water.
U.S. Pat. No. 5,141,536 describes a glycol dehydration apparatus having a shell-and-tube countercurrent condenser attached to the water vapor vent to condense volatile carbon compounds. The shell-and-tube condenser has an interior tube with an inlet and outlet for passage of the gaseous fluid, and a shell having an inlet and outlet for passage of wet glycol through an annular space between the shell and tube. While BTEX recovery efficiency of this patented apparatus is reportedly high, all the hydrocarbons existing in the vapor vent of the glycol reboiler may not be condensed in some cases, and vapors leaving the atmospheric vent downstream of the condenser are undesirably emitted to the environment.